Abstract
Persistence of populations through climate change demands biogeographic shifts of species, evolutionary adaptation of populations or local acclimatization of individuals. For long-lived, sessile foundation species that create ecosystem habitat such as forest trees or reef building corals, range shifts and evolution are predicted to be slow. As a consequence, the rate and scope of acclimatization in these species to future environmental conditions is central to understanding the impact of climate change. In recent decades, reef building corals have experienced global declines due, in part, to extreme bleaching events sparked by pulses of warm water exposure. However, corals in naturally warm environments can have higher resistance to bleaching temperatures than conspecifics in cooler microclimates, making these populations ideal test sites for research into the mechanisms of coral response to climate change. We use transplants between microclimates, physiological testing and transcriptomics to show that corals can acquire partial heat tolerance through gene expression changes in warm microclimates. Though acclimatization and fixed differences between populations contribute similarly to bleaching tolerance, acclimatization achieves the same phenotypic change in less than two years as would strong selection over many generations for these long-lived organisms. Acclimation and adaptation may both be important in future response of corals and coral reefs to gradual ocean warming, and including these process in predictive models will make for better evaluation of coral reef future.